It is conventional practice in the art field of packaging machines to employ a pneumatic flow compensating chamber, positioned along the feed path followed by the strip of wrapping material, of which the function is to absorb imbalances that can be created between the quantity of strip decoiled per unit of time from the respective roll, and the quantity of strip taken up in the same unit of time by the user station. Such flow compensating chambers are furnished with respective suction means capable of attracting the strip with a predetermined and constant force so that it is retained internally of the chamber as a running loop of variable length; in this way, the strip material can be maintained substantially at a constant tension as it is directed toward the user station, and the loop constitutes a reserve such as will compensate variations in the rate at which the strip is taken up by the user station.
In particular, the rate at which the strip decoils will be governed according to the length of the loop that is allowed to form progressively inside the flow compensating chamber; for example, an increase in the length of the loop means that the decoil rate is higher than the rate at which the strip is taken up by the user station, and accordingly, an adjustment must be made to the drive means controlling the rate of decoil from the roll.
The feed rate of the strip is also monitored directly along the path followed by the strip upstream of the cutting station, or alternatively downstream of the cutting station, in order to control the positioning of the discrete lengths generated by the cutting operation, also their timing relative to a user station lying downstream of the cutting station, and relative to the cutting station itself.
More particularly, in the case of materials comprising at least two component parts, consisting for example in two identical strips, or in a strip of clear and colourless plastic material and a ribbon of slender transverse dimensions decoiled from a roll and supplied as a continuous strip or in discrete lengths, embodied in the same type of material as the strip to which it will be bonded, it becomes necessary to verify the presence and/or the correct mutual positioning of the two component parts.
In prior art systems such checking functions are generally entrusted, by way of example, to optical or capacitive or inductive devices. These devices are not always reliable inasmuch as their performance characteristics can be rendered false in the case of transparent material, or may vary with the colour of the wrapping material, and can also be disturbed by layers of residual matter and dust deposited on the strip and on the devices themselves as the strip advances. It is also possible to use barrier photocells operating in the visible or the infrared spectrum, or a thickness check can be employed. These further methods allow only tight calibration margins, with the result that the system can be affected by instability.